1. Field of the Invention
Embodiments of the disclosure relate to a liquid crystal display and a global dimming control method thereof capable of controlling a luminance of backlight depending on a displayed image.
2. Discussion of the Related Art
A range of application for liquid crystal displays has gradually grown because of their excellent characteristics such as light weight, thin profile, and low power consumption. The liquid crystal displays have been used in personal computers such as notebook PCs, office automation equipments, audio/video equipments, interior/outdoor advertising display devices, and the like. The liquid crystal displays display an image using a thin film transistor (TFT) as a switching element. A backlit liquid crystal display occupying most of the liquid crystal displays controls an electric field applied to a liquid crystal layer and modulates light coming from a backlight unit, thereby displaying an image.
The image quality of the liquid crystal display depends on its contrast characteristic. There is a limit to an improvement of the contrast characteristic using only a method for controlling a data voltage applied to the liquid crystal layer to modulate a light transmittance of the liquid crystal layer. Thus, a global dimming control method for adjusting a luminance of backlight depending on an input image has been proposed, so as to improve the contrast characteristic. The global dimming control method calculates an average picture level (APL) of an input image and varies a turn-on duty ratio of a pulse width modulation (PWM) signal based on the APL, thereby adjusting the luminance of the backlight. For example, as shown in FIG. 1, the global dimming control method controls the turn-on duty ratio of the PWM signal to be about 100% based on the image having the APL of about 60%; controls the turn-on duty ratio of the PWM signal to be about 60% based on the image having the APL of about 5%; and controls the turn-on duty ratio of the PWM signal to be about 50% based on the image having the APL of about 4%. As above, the global dimming control method increases the luminance of the backlight as the APL increases, and vice versa. Hence, the global dimming control method improves dynamic contrast characteristic of the image measured between adjacent frames and at the same time reduces power consumption.
As shown in FIG. 2, the global dimming control method is generally implemented at the low APL, for example, at the APL equal to or less than about 10%. Images of the low APL (for example, equal to or less than about 10%) are generally images having characters on a black background or color images of the low APL. In this instance, if the duty ratio of the PWM signal corresponding to a dimming ratio decreases based on the low APL of the image, visibility of a chromatic region in a color image of the low APL having chromatic colors is greatly reduced as shown in FIG. 3. Hence, the image quality is degraded.